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Tractorsport Flowbench Forum Archive • View topic - Low cost digital readout of velocity and flow

Low cost digital readout of velocity and flow

Any discussion pertaining to data acquisition for a flowbench application. Not limited to computer related projects, this is a catch-all for anything non-water gage?

This will keep items of like interest easy to find on the forum.

Postby Tony » Fri Apr 25, 2008 10:04 pm

I have just received a pair of really great flow probes from Bruce, and have been thinking about how best to make myself a simple low cost velocity readout for them.

The first thing I did was to draw up a table of developed dynamic probe pressure versus velocity, and to do that, I found this dynamic pressure calculator on the internet:



Hint, for the program to work, Java script needs to be enabled on your computer.

To use the calculator, first enter the density of the fluid. Change the default value from (1,000 for water) to 0.0765 for air at 59 degrees Fahrenheit. Then click on imperial units (from the default SI units).

You can then enter the air velocity into the second field, and the software will calculate the developed dynamic probe pressure.

For instance enter 250 feet per second. And it will tell you that your velocity probe will develop 14.3 inches of water. At 500 feet per second the probe pressure will be 57.2 inches of water.

This is particularly interesting, in that it follows an exact inverse square law, the same as the orifice formula does for an orifice flow bench.
In other words:

Zero pressure = zero velocity (and zero orifice flow)
One quarter full pressure = half reference velocity (and half reference flow)
Full pressure = full scale reference velocity (and full scale reference flow).

The inverse square law velocity scale on a velocity probe manometer would look identical to the flow sale on a sloping manometer in an orifice bench, because it follows the exact same mathematical law.

A bit more thinking along these lines has led to a rather interesting and exciting discovery. There are many manufacturers around the world producing low cost digital voltmeter modules, both LED and liquid crystal versions, and these invariably use the same type of "chip" to make them work. Here is an example of a typical commercial LED voltmeter module:

Image

I have come up with a very simple and very accurate way to make these digital display modules display with a perfect inverse square law. In other words, you feed in a voltage exactly proportional to measured differential pressure, (from a pressure transducer) and the display will read correctly in either feet per second velocity, or CFM flow directly in the corrected units.

**************************
edit:

Further testing has shown that while this modification provides a smooth curve that at first glance appears to be an inverse square law, it is not a true inverse square law function, only an approximation. What fooled me was that it does produces the exact numbers at both ends, and right in the middle of the range, but it introduces a type of "S" distortion into the curve.

It reads low in the the low part of the range, correct in the middle, and slightly high at the upper end of the scale. Unfortunately the error is significant enough to sink this whole idea. I only discovered the problem after very carefully checking many more data points. More details about this later in the thread (top of page four).

*************************


These modules invariably use either the Intersil ICL7106 (LED type) or the ICL7107 (liquid crystal type), but otherwise they work exactly the same way. To make these modules display in inverse square law, all that is required to do this are two resistors that have an EXACT 3:1 resistance ratio, and cross coupling the pressure input signal to the reference input of the chip.

Image

Some suggested values for the two resistors might be 10k/30k, 11k/33k, 12k/36k, or 13/39k 1% metal film type.

The display module always reads 1,000 x input voltage at the chip, divided by the reference voltage at the chip.

Zero input voltage (zero pressure) displays "000" (chip i/p 0mV, chip ref 50mV) (reads 000)
+25mV input (quarter pressure) displays "250" (chip i/p +18.75mV, chip ref 75mV) (reads 250)
+100mV input (full scale pressure) displays "500" Chip i/p 75mV, chip ref 150mV) (reads 500)

That is with the reference voltage fixed at -50mV

By changing the fixed reference voltage, or the input voltage range, full scale readings other than "500" can readily be obtained, (up to "1999") but the inverse square law still always remains correct.
Also known as the infamous "Warpspeed" on some other Forums.
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Postby bruce » Fri Apr 25, 2008 10:39 pm

Gee I can see another project in my future!! :D
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Postby Tony » Fri Apr 25, 2008 10:47 pm

I though you already had an FP1 Bruce ?

This is more the ultra ultra low cost method of digital velocity measurement. But there is absolutely no reason why it cannot be made just as accurate.
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Postby 106-1194218389 » Fri Apr 25, 2008 11:17 pm

What I assume is we would also need a pressure transducer to convert to the electrical signal, am I correct? If we need a transducer would this one work or one from this web site?


And then could you make it work with this along with the digital read out?


John
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Postby bruce » Sat Apr 26, 2008 6:09 am

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Postby Tony » Sat Apr 26, 2008 7:28 am

O/k where I stand in all this is that I believe I can now see a way of coming up with something simple, practical, and relatively low cost, that others may wish to copy and build for themselves. I anticipate roughly around a hundred dollars worth of parts may be involved to build this, but we shall see.

There is still a fair way to go, I only came up with the concept yesterday and tested a prototype display today. It certainly converts linear pressure voltage signals into correct velocity numbers, or correct flow numbers with great precision. That is a very important first step.

The next step is to adapt a low cost commercial differential pressure transducer and connect it up to a suitable amplifier to get a suitable electrical pressure signal to drive the display. This is not as straightforward as it sounds, but it is not impossible. There will be problems to overcome with drift and temperature stability that need to be looked into.

As this all evolves I will post circuit diagrams for anyone to copy as a free service to other Forum members.

This circuit is NOT COMPUTER BASED. There is no software it will be just a small box with two pressure ports and a digital display on the front that displays air velocity in feet per second. Or flow in CFM for an orifice bench.

It will have one screwdriver adjustment to set the display to zero with no pressure applied, and a second screwdriver adjustment to set the full scale reference velocity against a water manometer.
If used as an orifice flow readout, the displayed CFM flow can be checked and adjusted against a known calibrated test orifice.
Once these two points are set, it should read very accurately over the whole range.

That is the way I see all this heading at this early stage.

This in no way competes with a proper fully featured flow computer.
It is just an ultra low cost do it yourself digital readout project for the benefit of the flow bench hobbyist.
Also known as the infamous "Warpspeed" on some other Forums.
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Postby bruce » Sat Apr 26, 2008 9:29 am

"There is no more formidable adversary than one who perceives he has nothing to lose." - Gen. George S. Patton
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Postby 115-1172523331 » Sat Apr 26, 2008 11:42 am

Thanks, Tony! I have been trying to figure out DAQ for a while and this could be the start to my "DAQ for Dummies" lessons: Lesson 1 - Displays! :D

I took a quick look online and found this (and other) pressure transducers. You mentioned "temperature stability" and this one has "temperature compensation" built in? Anyway, I will be watching and hoping!

Just to show how things work out, your dynamic pressure calcs made me think about adding a second tube next to the scale on my vertical manometer and having a dedicated velocity probe on the board! Larry M.'s velocity discussion in another part of the forum says 350 +/- is a "max" and I already have that height for my depression so "another project" (as Bruce would say)! -- Doug
115-1172523331
 

Postby 106-1194218389 » Sat Apr 26, 2008 12:04 pm

Good find on the tranducer Doug! I was looking for transducers and as you could see the ones I found were kind of expensive for a DIY project. Tony, I and I know everybody thanks you for your effort here. I wish I was better at electonics. This forum proves what can be done when the talents of many are combined. We all cannot know everything but if we work together we can accomplish great things. I really feel this is what life was meant to be. A combined effort so we can learn to also get along. Thanks you guys.

John
106-1194218389
 

Postby hdwgfx » Sat Apr 26, 2008 12:14 pm

If I am understanding this correctly, this would be a digital manometer capable of reading with the same accuracy as the high dollar digital units but just doesn't output the info. to a computer, we could still use Larry's workbook to manually enter the info., is the correct or am I all wrong??

This is awesome Tony! :D
Thanks,
david
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Postby bruce » Sat Apr 26, 2008 12:57 pm

Anybody (Tony?) have a problem if I add a new category for flowbench DAQ? I can then move this post over there so we keep all the DAQ discussion in one area.



Edited By bruce on 1209235500
"There is no more formidable adversary than one who perceives he has nothing to lose." - Gen. George S. Patton
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Postby Tony » Sat Apr 26, 2008 6:47 pm

Also known as the infamous "Warpspeed" on some other Forums.
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Postby bruce » Sat Apr 26, 2008 7:45 pm

Would this be what we will be looking for?





Edited By bruce on 1209254735
"There is no more formidable adversary than one who perceives he has nothing to lose." - Gen. George S. Patton
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Postby Tony » Sat Apr 26, 2008 8:08 pm

Also known as the infamous "Warpspeed" on some other Forums.
Tony
 
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Joined: Sat Dec 03, 2005 12:34 pm
Location: Melbourne, Australia

Postby bruce » Sat Apr 26, 2008 8:24 pm

Here is a good link to the Motorola sensors that are "possibly" being used in the flow computers:

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